Usually, an oxide superconducting wire is produced through the powder-in-tube (PIT) method. More specifically, a multifilament oxide superconducting wire having at its inside a plurality of filaments made of oxide superconducting material is produced through the following method. First, a powder composed mainly of a precursor of an oxide superconductor is heat-treated. The powder is filled into a second sheath made of metal. A drawing operation is performed to obtain a monofilamentary wire. A plurality of monofilamentary wires are inserted into a first sheath made of metal. A drawing operation is performed to obtain a multifilamentary wire, which is then subjected to a rolling operation. Finally, the rolled wire is heat-treated to complete the production.
Generally, the production of an oxide superconducting wire through the PIT method has so far been performed as described below. As shown in FIG. 5, a round monofilamentary wire 14 is produced by covering a filament 2, which has a circular cross section, with a second sheath 3 having the shape of a circular tube. Alternatively, as shown in FIG. 6, a hexagonal monofilamentary wire 15 is produced by covering a filament 2′, which has a hexagonal cross section, with a second sheath 3 having the shape of a hexagonal tube. A plurality of round monofilamentary wires 14 or hexagonal monofilamentary wires 15 are inserted into a first sheath 6 (for example, in the case of the filament having a circular cross section, 50 to 200 wires are inserted) (in FIG. 5, to avoid the diagram from becoming complicated, the number of wires is reduced). The first sheath is drawn to produce a precursor wire 5. The precursor wire 5 is rolled and then heat-treated to complete the production of the oxide superconducting wire. FIG. 7 is a diagram showing a step of inserting the predetermined number of round monofilamentary wires 14 into the first sheath 6 in the production process of the precursor wire 5 shown in FIG. 5 described above.
In the oxide superconducting wire produced by using the precursor wire of an oxide superconducting wire shown in FIG. 5 or 6, however, because the cross-sectional shape of the filament is isotropic such as circular or hexagonal, the filament is thick, so that the orientation ability of the crystal of the oxide superconductor cannot be increased. As a result, it has been difficult to offer an oxide superconducting wire having high critical current density.
As the method for increasing the critical current density of an oxide superconducting wire, the following methods have been attempted, for example. A first method provides a core material at the inside of the superconductor (Patent Literature 1). A second method provides a core material having a circular cross section in the center portion, prepares a tape-shaped monofilamentary wire, and attaches the wire longitudinally on the core material such that the entire surface of the wire is brought into intimate contact with the core material (Patent Literature 3). A third method places a plurality of ribbon-shaped filaments in a stabilized matrix having a cross section of a circle or a rotationally symmetric polygon and having a center portion formed with a stiff material (Patent Literature 2).